A liquid crystal element is conventionally produced by the following steps of: forming electrodes, alignment films, etc., and spacers when needed, on each of two substrates made of glass or the like; producing a cell by laminating these substrates in parallel with each other while keeping a minute space therebetween; and injecting liquid crystal into the space between the substrates.
Before these substrates are laminated, as shown in FIG. 12, a sealing agent 54 is applied around the circumference of a liquid crystal filling area 53 in at least one of the substrates (herein, substrate 52) except portions 51 which will be used as an inlet through which the liquid crystal is injected.
A conventional liquid crystal injecting device for injecting the liquid crystal into the cell and a conventional liquid crystal injecting method are shown in FIG. 13. More specifically, a liquid crystal reservoir 62 withholding the liquid crystal and a cell 63 are provided in a pressure reducing bath 61. After an internal pressure of the pressure reducing bath. 61 is reduced, the liquid crystal is supplied to an inlet of the cell 63 by dipping the inlet into the liquid crystal reservoir 62, for example. Then, the liquid crystal is injected into the cell 63 by a difference in pressure between the interior and exterior of the cell 63 given by increasing the internal pressure of the pressure reducing bath 61 as high as or higher than atmospheric pressure.
Another example of a conventional liquid crystal injecting device and a conventional liquid crystal injecting method are shown in FIG. 14. More specifically, an LC (Liquid Crystal) inlet 72 and an air outlet 73 are made through one of the opposing substrates which form the cell (herein, substrate 71). After displacing air in the cell through the LC inlet 72 and air outlet 73 by pressure reducing devices 77 and 78 which are equipped with vacuum pumps 75 and 76, respectively, liquid crystal 74 is injected into the cell through the LC inlet 72.
In case of injecting liquid crystal having a smectic phase, and therefore a high viscosity, such as ferroelectric liquid crystal, the liquid crystal is generally heated before the injection, so that the liquid crystal becomes less viscous and more fluid when injected. This process demands three following conditions:
(1) the cell of the liquid crystal element should be filled with the liquid crystal without leaving any unfilled area; PA1 (2) the substrates forming the cell of the liquid crystal element should not be deformed, and the cell of the liquid crystal element should not be damaged; and PA1 (3) each step in the injecting process should not take long. PA1 a liquid crystal applying step of applying liquid crystal to at least one of the openings and a vicinity thereof; PA1 a cell's internal pressure reducing step of releasing air from an interior of the cell through at least one of the openings to lower an internal pressure of the cell than an external pressure thereof; PA1 a cell temperature setting step of setting a temperature of the cell to a temperature at which the liquid crystal to be injected therein takes one of a nematic, cholesteric, and isotropic phase; PA1 a liquid crystal injecting step of injecting the liquid crystal into the cell through the opening to which the liquid crystal has been applied while air being kept released from the cell; PA1 a liquid crystal re-injecting step of applying the liquid crystal to all the openings of the cell, and re-injecting the liquid crystal through all the openings at room temperature or above under atmospheric pressure or below; and PA1 a cell cooling step of cooling the cell to room temperature after the liquid crystal has been re-injected into the cell. PA1 at least one liquid crystal injecting section having a vessel capable of increasing and decreasing an internal pressure thereof, and a pressure adjusting member for adjusting an internal pressure of the cell independently of an external pressure of the cell through the opening of the cell, said cell being sealed airtight in said vessel; and PA1 an airtight sealing bath capable of increasing and decreasing an internal pressure thereof, and enclosing at least one liquid crystal injecting section therein. PA1 at least one liquid crystal injecting member having an airtight sealing member for separately sealing one of the openings used to inject the liquid crystal into the cell and one of the openings used to release air from an interior of the cell, and a pressure adjusting member for adjusting pressures separately at an airtight sealing portion of the opening used to inject the liquid crystal into the cell and at an airtight sealing portion of the opening used to release air from the interior of the cell; and PA1 an airtight sealing bath capable of increasing and decreasing an internal pressure thereof, and enclosing at least one liquid crystal injecting section therein.
The first condition is required because if an unfilled area is left in the resulting liquid crystal element, not only the outward appearance is deteriorated, but also an operating life of the liquid crystal is shortened as the liquid crystal element is repeatedly driven.
The second condition is required for the following reason. That is, if the cell of the liquid crystal element is deformed, a cell gap between the substrates forming the cell varies, in response to which a threshold voltage is varied, the electrodes formed on the opposing substrates develop a short circuit, the alignment state of the liquid crystal is changed, etc., thereby making it impossible to attain satisfactory display quality. In particular, in case of the ferroelectric liquid crystal or anti-ferroelectric liquid crystal, since the cell gap is generally as small as 2 .mu.m, the aforementioned inconveniences can readily occur with a very small variance. Moreover, when the substrate is deformed considerably, there arises another problem that the cell forming the liquid crystal element is readily damaged.
The third condition is required for the following reason. That is, as previously mentioned, since the liquid crystal injecting process involves the vacuuming step and heating step, if this process takes too long, some particular components contained in the liquid crystal may evaporate and the composition of the liquid crystal may be changed, or the liquid crystal may be deteriorated by heat. Further, the longer the liquid crystal injecting process, the lower the producing efficiency, thereby causing another problem that the manufacturing costs are undesirably increased.
To satisfy these conditions, many patent applications relating to the injecting method of the smectic liquid crystal have been filed, and some of them have been granted patents.
For example, Japanese Patent No. 18007010 discloses a liquid crystal injecting method. According to this injecting method, a cell applied with the smectic liquid crystal near its opening is placed in a pressure-resistant vessel, and air in the pressure-resistant vessel is displaced. Then, the cell is heated, so that the opening of the cell is closed with the heated and thus having become fluid smectic liquid crystal. Subsequently, reducing of the internal pressure of the vessel is stopped.
However, according to the injecting method of the above Japanese Patent, air in the cell is not readily displaced at the very initial stage of the air-displacing process, and for this reason, a difference in pressure between the interior and exterior of the cell is so small that the cell may be broken during the liquid crystal injecting step. This problem occurs more frequently with the cells of a larger size. This problem may be eliminated by reducing an internal pressure of the vessel more slowly. However, from the standpoint of satisfying the condition of shortening the injecting process, this solution is not preferable because a displacement rate decreases with increasing sizes of the cells.
In addition, the cell is heated while the ferroelectric liquid crystal is injected therein. However, if the injection method of the above Japanese Patent is adopted in this case, the substrates forming the cell may be deformed by heat, and the cell readily warps, and possibly, to the extent that it is broken. This problem also occurs more frequently with the cells of a larger size.
Moreover, since air in the cell is not displaced satisfactory, air left in the cell may be compressed when the liquid crystal is injected, and left as bubbles in the end. When air in the cell is not displaced satisfactory, a non-polymerized monomer of a high molecular film and moisture contained in color filters provided on the substrates forming the cell are left in the cell in the form of a gas, thereby making it impossible to inject the liquid crystal into the cell to its full.
Further, the smectic liquid crystal increases its volume (expands) when heated during the injecting step, and reduces its volume (contracts) when cooled later. Thus, even when the smectic liquid crystal is injected fully into the cell at the injecting step, cavitation may occur due to the volume loss (contraction) of the smectic liquid crystal in the cooling step, thereby causing deficient injection of the liquid crystal.
A method disclosed in Japanese Laid-open Patent Application No. 297386/1993 (Japanese Official Gazette, Tokukaihei No. 5-297386) is suitably used to cut the injection time shorter and eliminate residual bubbles. According to this method, at least one injection hole and at least one air-releasing hole are provided at the circumference of the cell, and air in the cell is displaced through these holes, after which the liquid crystal is injected into the cell through the injection hole while air is kept released from the air-releasing hole.
Also, a method disclosed in Japanese Laid-open Patent Application No. 220550/1996 (Japanese Official Gazette, Tokukaihei No. 8-220550) is suitably used to eliminate the cavitation. According to this method, after the phase of the smectic liquid crystal which has been injected into the cell is restored to isotropic phase, the cell is filled with another smectic liquid crystal with an application of pressure.
However, in case of the smectic liquid crystal which has to be heated when injected, it is very difficult to eliminate the cavitation by the method of Japanese Laid-open Patent Application No. 297386/1993. Also, once the bubbles are left in the cell, they can not be eliminated by the method of Japanese Laid-open Patent Application No. 220550/1996 even if a pressure is applied.
Further, the cavitation can not be eliminated completely by the method of Japanese Laid-open Patent Application No. 220550/1996 for the following reason. That is, the flowing rate of the liquid crystal, flowing into the space between the opposing two substrates forming the cell, starts to drop as its top approaches to the air outlet, thereby extending the injection time. Thus, unfilled areas are readily produced near the air outlet. Moreover, since the cell filled with the liquid crystal is cooled to produce the liquid crystal element, the cavitation may occur in the cell of the liquid crystal element due to the volume loss of the liquid crystal.